1. Field of the Invention
The subject invention is directed to a process for preparing alkyl nitrites, particularly methyl nitrite, and a reaction vessel for carrying out the process.
2. Description of Related Art
Alkyl nitrites, i.e., esters of nitrous acid, have been found useful in a variety of areas including additives to motor fuels, stabilizers for vinyl compounds such as spasmolytic agents, reagents for diazotization and reagents for chemical synthesis. Processes for preparing alkyl nitrites can be found, inter alia, in U.S. Pat. Nos. 4,229,591; 4,353,843 and 4,629,806 and in Japanese Application No. 53-8268. The process for forming alkyl nitrites (referred to herein as the nitrite process) may be understood more fully by reference to the following equations: EQU (1) 2NO+O.sub.2 .fwdarw.2NO.sub.2 EQU (2) NO.sub.2 +NO.revreaction.N.sub.2 O.sub.3 EQU (3) ROH+N.sub.2 O.sub.3 .fwdarw.RONO+HONO EQU (4) ROH+HONO.fwdarw.RONO+H.sub.2 O EQU (5) N.sub.2 O.sub.3 +H.sub.2 O.fwdarw.2HONO EQU (6) 2NO.sub.2 .revreaction.N.sub.2 O.sub.4 EQU (7) ROH+N.sub.2 O.sub.4 .fwdarw.RONO+HNO.sub.3 EQU (8) N.sub.2 O.sub.4 +H.sub.2 O.fwdarw.HONO+HNO.sub.3
wherein R represents a methyl or ethyl group.
The desired reaction sequence for the formation of alkyl nitrite occurs via Reactions (1)-(4). The sum of these reactions yields as the overall process reaction: EQU (I) 2ROH+2NO+1/2O.sub.2 .fwdarw.2RONO+H.sub.2 O
Reaction (5) takes place because the water formed in Reaction (4) can react with dinitrogen trioxide (N.sub.2 O.sub.3). Reaction (5) can be tolerated provided enough alcohol is supplied to react with substantially all of the nitrous acid formed in Reaction (5) according to Reaction (4) yielding alkyl nitrite and additional water.
Reactions (6) through (8) are undesired since they lead to the formation of nitric acid, a compound which subsequently must be separated from product alkyl nitrite. Further, these reactions consume nitric oxide in forming undesired nitric acid. In order to reduce production of dinitrogen tetroxide (N.sub.2 O.sub.4), via Reaction (6), the gas phase concentration of NO.sub.2 should be minimized relative to that of NO. In this way, N.sub.2 O.sub.3 is preferentially formed instead of N.sub.2 O.sub.4. A relatively high NO to NO.sub.2 ratio can be maintained by initially supplying a molar excess of NO relative to O.sub.2, as indicated by the stoichiometry of Reaction (I), i.e., greater than 4 moles NO per mole O.sub.2. In other words, to enhance production of alkyl nitrites such as methyl nitrite or ethyl nitrite, it generally is preferable to provide NO in a molar excess, preferably in such an amount that substantially all O.sub.2 is consumed.
Vapor state formation of alkyl nitrite (nitrite process) by the general procedure described above preferably is coupled and correlated with vapor state formation of dialkyl oxalate from alkyl nitrite and carbon monoxide (oxalate process) in an integrated production cycle so as to provide an overall vapor state process (nitrite-oxalate process) that is cyclic in operation, e.g., see U.S. Pat. No. 4,629,806. Such a process is advantageous with regard to limiting the formation of by-products, ease of operation and production efficiency. Vapor state formation of dialkyl oxalate is conducted by contacting carbon monoxide and alkyl nitrite in a carbonylation reaction zone in the presence of a solid catalyst. The main reaction is illustrated by the following equation: ##STR1## wherein R represents a methyl or ethyl group.
Preparation of dialkyl oxalates is of particular interest to the chemical industry because of the varied uses of these compounds. These diesters may serve as starting materials for the preparation of alkylene glycols such as ethylene glycol, a valuable commercial chemical which finds application in deicing fluids, antifreeze, hydraulic fluids and in the manufacture of alkyd resins, solvents, and polyester fibers. These diesters also are useful as intermediates in preparing dyes, pharmaceuticals, and the like.
As evident from the equation representing Reaction (II), for every mole of alkyl nitrite consumed, a mole of nitric oxide is generated. Nitric oxide thus formed may be recycled and used as a starting material for forming alkyl nitrites according to Reaction (I), thus completing the nitrite-oxalate reaction cycle. Dialkyl oxalate produced in the carbonylation reaction zone can be purified and recovered as product or further reacted, for example, by contacting it with hydrogen in a hydrogenation reaction zone to produce ethylene glycol.
To provide an efficient process for preparing alkyl nitrites, a number of performance criteria must be considered and satisfied.
First, oxygen conversion preferably should be as close to 100 percent as is possible (i.e., the amount of oxygen exiting the alkyl nitrite reactor preferably is minimized) without significantly adversely affecting other reactor performance characteristics. It also is preferred that substantially all higher nitrogen oxides, i.e., oxides of nitrogen other than nitric oxide, be consumed in the alkyl nitrite reactor.
Second, the efficiency of converting nitric oxide to alkyl nitrite, i.e., the percentage of nitric oxide converted to alkyl nitrite, the desired product, is maximized in the alkyl nitrite reactor while formation of undesired products such as nitric acid is minimized.
It also is preferred that substantially all water and nitric acid produced in the alkyl nitrite reactor be removed in a liquid tails stream by providing a scrubbing agent which scrubs water and nitric acid from the gaseous product stream. The amounts of water and nitric acid present in the gaseous product stream from the alkyl nitrite reactor thus are minimized. Conversely, the amount of alkyl nitrite, the preferred product, present in the liquid tails stream from the alkyl nitrite reactor similarly is minimized.
The amount of scrubbing agent required to provide the separation necessary to satisfy the above discussed requirements preferably is minimized since the use of excess scrubbing agent material is uneconomical.
Finally, Reaction I is highly exothermic, and it is necessary to remove heat from the alkyl nitrite reactor.